1. Field of the Invention
The present invention relates to a screen device of a screen printing machine, and in particular to the art of adjusting a position of a screen plate including a screen and a screen frame, in a direction parallel to the screen.
2. Related Art Statement
Generally, a screen plate which is employed in a screen printing machine includes a screen and a screen frame to which the peripheral portion of the screen is fixed. When a screen printing operation is carried out, the screen plate is fixed to a frame-support table of the screen printing machine, after a position of the plate relative to the table in a direction parallel to the screen is adjusted. The position of the frame-support table relative to an object to be printed, such as a printed circuit board, is not adjusted, but the position of the screen plate relative to the object is adjusted by adjusting the position of the plate relative to the table.
For example, Japanese Patent Application laid open for opposition, under Publication No. 3(1991)-33109, discloses a screen device which includes a screen plate and a screen-plate frame to which the screen plate is detachably attached. The screen-plate frame is detachably attached to a frame-support table, and the position of the screen-plate frame relative to the frame-support table is adjusted to adjust the position of the screen plate relative to an object to be printed. The frame-support table includes a frame-support portion which can support the screen-plate frame, and a cylinder-retain portion which retains fixing cylinders. In a state in which the screen-plate frame is placed on the frame-support portion of the frame-support table, the frame is pushed against the frame-support portion by the fixing cylinders, and thus is fixed to the table.
The prior screen device additionally employs a lifting means for lifting, when a position adjusting operation is carried out, the screen-plate frame up and thereby lightly moving the frame relative to the table. The screen frame needs to be so large and rigid as to stretch the screen without slack and maintain an appropriate tension of the screen. A unit consisting of the screen plate including the very screen frame, and the screen-plate frame holding the screen plate, is heavy and, when the unit is moved on the frame-support table, a large frictional force is produced between the screen-plate frame and the frame-support table. Accordingly, it is difficult to move the frame on the table, in particular, correct small positional errors of the frame relative to the table.
To solve this problem, the lifting means is operated, when a position adjusting operation is carried out, to lift the frame up from the frame-support portion of the table, thereby reducing the frictional force. One example of the lifting means comprises a plurality of compression coil springs which are provided between the table and the frame and which elastically raise the frame. Another example of the lifting means comprises an air-pressure-type lifting device which supplies a pressurized air to create a space between the frame and the frame-support portion of the table, thereby lifting the frame up from the frame-support portion.
In the case of the compression coil springs, it is difficult to zero the frictional resistance produced between the frame and the table. Although the frame must not contact the frame-support portion nor cylinder-support portion of the table to zero the frictional resistance, the respective elastic forces of the compression coil springs cannot be precisely equal to each other, i.e., may differ from each other, and additionally may timewise change. Therefore, if it is intended to lift surely the frame up from the frame-support portion of the table, the frame may contact the cylinder-support portion of the table that is provided above the frame-support portion. On the other hand, if it is intended to prevent the frame from contacting the cylinder-support portion, it is impossible to separate completely the frame from the frame-support portion. Thus, some frictional resistance remains between the frame and the table, which does not allow the frame to be lightly moved relative to the table. Meanwhile, the air-pressure-type lifting device needs a complex structure and occupies a large space, which leads to increasing the production and running costs of the screen device. These problems with the lifting means are also encountered in the case where no screen-plate frame is employed, that is, the screen plate is directly fixed to the frame-support table after the position of the plate relative to the table is adjusted.
If the screen frame is formed of a light material such as aluminum, the total weight of the frame is reduced. However, the aluminum frame suffers insufficient rigidity, and cannot give the screen an appropriate tension. In addition, if some external force is exerted to the screen plate when a position adjusting operation is carried out, some strain might be produced in the screen frame. To solve this problem, it is needed to employ the screen frame which has a large second moment of area. Thus, the screen frame needs to have a large size and a large weight.